The present invention relates to a transferring device for transferring on a record paper a toner image formed onto an image forming member, such as a photosensitive drum and an image forming apparatus utilizing such a transferring device.
In an image forming apparatus, such as an electrophotographic printing apparatus or a copying device, an electrostatic latent image is formed onto a photosensitive member or body, the electrostatic latent image is developed to make a toner image, the toner image is transferred onto a recording medium, such as, a record paper and the transferred image is subjected to a fixing step to form a hard copy. Various kinds of transferring devices for transferring the toner image formed onto the photosensitive body are well known, for example, the belt transferring device disclosed in Japanese Patent Application Laid-open No. 61-117583. In this known belt transferring device, a transfer belt formed by a dielectric material is stretched between two drive rollers, the transfer belt is charged at a uniform potential by a corona charger, the record paper carried in synchronism is electrostaticaly attracted onto the transfer belt, and then a toner image transfer process is performed while carrying the record paper under the state of holding it to the transfer belt. After transferring the toner image onto the record paper, charges on the surface of the transfer belt are uniformly removed by a charge remover which is composed of a high A.C. supply source and a corona charger and then the transfer belt stands by for next transferring step. That is, the transfer belt is subjected to a charge removing process before every charging to decrease the potential of the transfer belt surface to a given surface potential, after which the transfer belt is again subjected to a charging process to always make the potential of the transfer belt a given certain surface potential.
In the known belt transferring device, the record paper is carried under the state of holding it on the transfer belt and a toner image is transferred onto the record paper with the use of charges on the transfer belt, so that the carrying and the transferring for the record paper may be performed with high reliability.
In order to record an image having uniform image density on the record paper, it is always necessary to hold the charging potential of the transfer belt at a certain potential. Therefore, in the conventional belt transferring device, an A.C. charge remover is arranged between the transferring position and the charging position of the transfer belt, and then once the potential of the transfer belt surface is decreased to a given potential by the A.C. charge remover, the transfer belt surface is again charged to a certain potential. In order to perform A.C. charge removing, it is necessary to provide a high A.C. supply source and a corona charger, so that manufacturing cost becomes high and the construction becomes complicated.
Also, a copying machine and a printing apparatus of duplex type have been put into pratical use which are capable of recording image on both a surface and a back surface of the record paper. In this image forming device of duplex type, toner image transferred on one surface of the record paper is fixed with a heat fixing device such as heat roll fixing device, and then the record paper is turned upside down so as to transfer a toner image on the back surface of the record paper. When the record paper passes through the heat fixing device, however, the percentage of moisture content of the record paper becomes decreased substantially, so that the electrostatic capacity becomes remarkably small. Therefore, in an image forming device using the conventional transferring device, when the image recording is performed on the back surface of the record paper, an electric field caused between a photosensitive drum and the record paper at transferring and for effecting the transfer becomes decreased substantially so that the transferring efficiency becomes decreased greatly. Thus the image density also becomes decreased, thereby causing a decrease of image quality, or a generation of edging phenomenon.
The record paper has large hygroscopic property so that the percentage of moisture content of the record paper is largely changed under the ambient environment.
More particularly, the specific resistance of the record paper becomes greatly decreased under high humidity, and thus the transferring efficiency becomes decreased, resulting in a decrease of image density.
The image recording is also performed on record papers having various different thicknesses. In this case the distance between the transfer belt surface and the photosensitive drum surface is changed in accordance with the thickness of the record paper to be used so that field intensity for attracting the toner image to the record paper is changed depending on whether thick record paper or thin record paper is used, thereby decreasing the transfer efficiency in case of recording the image on the thick record paper, resulting in a decrease of image density.